1. Field of the Invention
This invention relates to improved polymeric orthopaedic compositions such as particulate powders, continuous media for bone cements, finished bone cements and orthopaedic implant coatings which are characterized by an amount of physiologically acceptable polymeric matrix with sized fibers dispersed in the matrix such that the sizing layer on the fibers is chemically joined to the surface of the fibers and to the matrix material. In preferred forms, sized radiopaque particles can also be added to the orthopaedic compositions.
2. Description of the Prior Art
Starting in the mid-1930s and continuing through the middle part of the 1950s, the orthopaedic implantation of artificial femoral heads, i.e., the ball of the hip joint, was achieved primarily by impaction of the stem of the implant into the medullary (marrow) cavity of the femur. Although this procedure achieved some degree of success in that it increased the mobility of certain patients suffering from hip joint deterioration or malfunction, the procedure had many shortcomings. The principal problem encountered was traced to loosening of the implant in a relatively few years, thus compromising the patient's mobility and oftentimes causing significant pain.
In the late 1950s, Dr. John Charnley in England proposed use of a mortar or grouting agent for cementing an orthopaedic appliance to a patient's bone structure using polymethylmethacrylate (PMMA) as the grout agent. The PMMA mortar material served to fill all of the spaces between the stem of the implant and the surrounding bone. In particular, it was determined that the PMMA grouting agent could be forced into the tiny interstices of the porous bone. This resulted in a mechanical lock of the grout agent to the bone. Another advantage of the PMMA system was the fact that mobilization of the patient following implantation could be accomplished at an earlier date following surgery, and the functional lifetime of the implant was greatly enhanced.
The success obtained in hip surgery using PMMA encouraged orthopaedic surgeons to develop implant attachment techniques for other prostheses such as the acetabular or cup side of the hip joint and for the femoral, tibial and patella components of total knee replacements. Although the life time of these prostheses has markedly improved and the range of patients for which the surgery is deemed appropriate has greatly increased, difficulties still remain. There is still a tendency for the cement mantle which surrounds the prosthesis to fail by brittle fracture and fatigue and, thereby, to lose its ability to transmit load from the implant to the bone structure. This loss of load transfer capability in turn was found to cause loosening of the prosthesis with concomitant joint dysfunction, failure of the metal prosthesis itself by virtue of loss of support, and finally pain during patient activity because of gross movement of the prosthesis.
Efforts to remedy these problems have not met with a great deal of success. One attempt involved increasing the thickness of the cement mantle. Another proposal chose to proceed in the opposite direction by significantly decreasing the thickness of the mantle. Thorough cleaning and lavage of the bony surfaces in order to promote interdigitation and better mechanical locking of the cement with the interstices of the bone structure was also tried. Other researchers suggested pressurization of the cement during insertion to further promote introduction of the cement into the interstitial spaces of the bone. Grouting guns were used to eliminate seams and laps in the cement mantle. Finally, incorporation of strong reinforcement fibers in the cement was tried, including 316 type stainless steel wires, cobalt-chromium-molybdenum implant alloy wires, glass fibers, aramid fibers and polyethylene terephthalate (PET). These prior procedures did not adequately solve the problem for two basic reasons. First, prior researchers failed to appreciate that although cement reinforcement was desirable, their reinforcements (except PET) were so rigid and stiff that they actually bridged bone interstices impeding full filling of the interstitial cavities. Secondly, these same researchers made no attempt to chemically couple the fibers to the matrix polymer. Consequently, little or no load was transferred to the fibers and they were unable to participate in load bearing. Accordingly, the fibers conferred little improvement in strength or crack resistance to the cement.
An exemplary effort in this respect was the attempt in the early 1980s to reinforce bone cement materials with carbon fibers. The fibers were added to the powder mix so that upon consolidation of the bone cement, the fibers were only dispersed in the matrix phase but not in the original PMMA particles. Carbon fiber addition had little, if any, beneficial effect on the toughness of the bone cement because the fibers were not coupled to the matrix. Further, these very stiff fibers greatly impeded the intrusion of the cement into the small bony interstices. This product has subsequently been removed from the market.